Field of the Invention
The invention lies in the automotive arts. In particular, the invention relates to a method of specifying the injection-pressure setpoint value in accumulator injection systems for supplying fuel in internal combustion engines.
Use is increasingly made of accumulator injection systems for supplying fuel in internal combustion engines. Such accumulator injection systems operate at very high injection pressures. Such injection systems are known as common-rail injection systems (for diesel engines) and HPDI injection systems (for spark ignition Otto engines). These injection systems are distinguished by the fact that the fuel is fed, using a high-pressure pump, into a pressure accumulator which is common to all cylinders and from which the injectors or injection valves at the individual cylinders of the engine are supplied. The opening and closing of the injection valves is as a rule controlled electromagnetically. The injected quantity of fuel is proportional to the opening duration of the injection valve and to the system pressure or injection pressure which is measured by means of a pressure sensor on the pressure accumulator.
The injection pressure in such a system is independent of the engine speed and therefore constitutes an additional variable which makes it possible to inject the fuel in dependence on the demand. The injection pressure has a considerable influence on the combustion process in the cylinder, by means of, for example, the atomization of the fuel as a function thereof. By raising the injection pressure in the lower rotational speed range it is possible to improve the exhaust gas values, for example. Generally the procedure is always to prescribe an injection pressure which is adapted to the engine operating point and the operating state, in order to obtain combustion which is at an optimum in terms of the emission of pollutants, the combustion noise and the generation of torque.
In the prior art, the injection pressure was specified, in particular in the case of the common-rail system, solely by means of a single characteristic diagram which is addressed via the currently injected quantity of fuel and the current engine speed. Transition states which result, for example, when accelerating out of a transient, non-steady engine operating state, cannot be adequately taken into account in such a procedure.